Several tests have been proposed to probe whether the compact objects in a binary are BHs or ECOs.
One of them is using echoes to distinguish the remnants of binary merger from BHs, which has initiated rigorous modelling and search for those features in GW data [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}. Construction of waveforms of binary ECOs has also begun [15]}, [16]}. Measurement of tidal deformability (TD) [17]}, [18]}, [19]}, [20]} and spin-induced multipole moments [21]}, [22]}, [23]}, [24]}, [25]}, [26]}, [27]}, [28]} from the late inspiral can also be used to test the presence of black holes. Nature of the compact objects can be probed with
GWs emitted due to superradiant instability as well [29]}, [30]}.
